\documentclass[11pt, oneside]{article}
 \begin{document}
 \begin{table}
 \begin{tabular}{c}
 \begin{tabular}{rrrr}   \hline min ATT & average ATT & median ATT & max ATT \\    \hline -26991.1 & 1041.2 & 944.0 & 21057.5 \\    -5438.3 & 652.8 & 726.0 & 2007.7 \\    287.4 & 1031.1 & 999.7 & 1931.0 \\     \hline \end{tabular} 
 \\\\ \begin{tabular}{rrrr}   \hline min Std. Error & average Std. Error & median Std. Error & max Std. Error \\    \hline 502.7 & 997.8 & 720.4 & 18752.4 \\    452.4 & 863.0 & 728.9 & 3414.0 \\    461.6 & 529.6 & 524.2 & 681.3 \\     \hline \end{tabular} 
 \\\\ \begin{tabular}{rrrr}   \hline min n & average n & median n & max n \\    \hline 4 & 298 & 280 & 594 \\    8 & 288 & 293 & 592 \\    360 & 550 & 552 & 713 \\     \hline \end{tabular} 
 \end{tabular}\caption{Distribution of estimated ATTs, their standard error and number of matched units for the different matching methods.}\end{table}
\end{document}